The present invention relates to a system and method of controlling vehicle steer-by-wire systems.
Steer-by-wire systems replace mechanical linkages between the steering wheel and the front road wheels of a vehicle with electrical wires and electronic components. Moreover, the mechanical linkages between the two front road wheels are eliminated in some kind of steer-by-wire systems. Instead, two independent actuators are installed on the vehicle, wherein each actuator independently actuates one of the front road wheels. This allows the two front road wheels be able to move independently from each other. Thus, a steer-by-wire system may be regarded as having two parts: a steering wheel sub-system and a road wheel sub-system. The electrical signals are translated through the wires to link the steering wheel sub-system to the road wheel sub-system with two independent front road wheels.
There is a need to provide a more flexible application environment within the steer-by-wire systems with two independent front road wheels to realize different left and right road wheel angle requirements such that the vehicle dynamics and safety can be improved. Steering functions, including different left and right road wheel angle requirements, need to be realized by utilizing the control system design.
Control system designers for such steer-by-wire systems face several challenges. First of all, a coupling between the left and right road wheel angles is experienced and a varying degree of coupling is produced with changing road conditions and vehicle dynamics. Typically, a change in an input control variable of a road wheel actuator will more than likely affect a proportional change in the directional angles of both left and right front road wheels. Thus, different left and right road wheel reference angles cannot be achieved by using independent left and right road wheel controller without consideration of road wheel angle coupling. It is a challenge for control system designers of steer-by-wire systems to control separately and independently each vehicle road wheel angle under an influence of coupling between road wheel angles.
Other challenges for control system designers include considering uncertain perturbations and severe non-linear characteristics of vehicle dynamics and actuator based steer-by-wire system. Typically, the vehicle dynamics change with road conditions, vehicle load, and external circumstances. A nonlinear relationship between road wheel actuator control input and road wheel angle output variables exists due to vehicle dynamics influence. The vehicle dynamics with uncertain perturbations and severe nonlinear characteristics may significantly affect the stability and performance of a steer-by-wire system of the vehicle. Moreover, the modeling accuracy of the controlled steer-by-wire system may not be sufficient to achieve the control performance specifications because of uncertainties and non-linear characteristics.
The present invention generally provides a system and method of controlling vehicle steer-by-wire systems with two independent front road wheels. The steer-by-wire system in accordance with the present invention may be viewed as having two parts: a steering wheel sub-system and a road wheel sub-system with two independent front road wheels. Electrical signals are transmitted via electrical wires to link the steering wheel sub-system to the road wheel sub-system. Main functions of the steering wheel sub-system are to provide a steering directional reference angle and produce an appropriate steering feel to a driver of the vehicle. Main functions of the road wheel sub-system are to establish tracking between road wheel angles and a steering wheel angle reference input which is provided by the steering wheel sub-system. These two subsystems are integrated in the steer-by-wire system to maintain alignment between the steering wheel and the road wheels of the vehicle and to implement vehicle steering functions.
A servo feedback control is introduced to the steer-by-wire control system design and is implemented in the steer-by-wire system in accordance with the present invention. This allows each of road wheel angles to track a respective road wheel input reference angle with minimum tracking error. The servo stiffness of roads wheels makes the vehicle to have the capability to reject the disturbances for the vehicle, such as wind gust disturbance.
To model a controlled road wheel system with two road wheel angle loop interactions, a multiple-input and multiple-output (MIMO) model description is applied to represent an inherent coupling characteristic between actuator control input variables and road wheel angle output variables.
The multivariable control with a multivariable controller is introduced to road wheel servo control system design and is implemented in the road wheel sub-system of the steer-by-wire system. The multivariable controller with decoupling characteristic reduces the affects of road wheel angle loop interactions. In other words, the present invention xe2x80x9cdecouplesxe2x80x9d the dependent road wheel angle variables and allows each road wheel to be controlled independently of each other by using a multivariable controller. The steer-by-wire system allows for an angle input variable for a corresponding road wheel to affect only the angle output variable of the corresponding road wheel rather than affecting both left and right road wheel angles.
By using a robust control strategy in the multivariable road wheel control system, the effect of uncertainty and disturbance may be reduced, and control system robust stability and performance may be improved. By using a gain scheduling control strategy in the multivariable road wheel control system, the gains of the controller changes automatically with the vehicle speed to compensate the gain change of the controlled road wheel system. Thus, the effect of non-linearity may be reduced and control system tracking performance may be improved.
In one embodiment of the present invention, a multivariable H∞ control is implemented as one example of a multivariable road wheel system control. The multivariable H∞ control design considers the uncertain perturbations and external disturbances existed in the road wheel subsystem. This provides a robust multivariable control for controlling the road wheel sub-system. The multivariable H∞ control design may consider the coupling of the controlled plant reduces the affects of road wheel angle interactions by decoupling design.
To obtain the different left and right road wheel reference angles, the reference command generator of the road wheel sub-system for the steer-by-wire system is implemented. The reference command generator receives a steering wheel angle and some vehicle variables and generates separate left and right road wheel reference angles. The variable steering ratios are implemented by using vehicle speed as a scheduling signal to obtain the different left and right road wheel reference angles. A calibration for left and right road wheel angles is performed according to vehicle geometry and steering performance requirements using vehicle variables.
The above-mentioned steer-by-wire systems with two independent front road wheels provide a more flexible application environment to realize different left and right road wheel angle requirements and to improve the vehicle dynamics. The control algorithms are implemented using a software program in a microprocessor-based steer-by-wire control module.